Joint with inherently limited torque level



y 2, 1968 I GEORGE s. WING 3,390,906

JOINT WITH INHERENTLY LIMITED TORQUE LEVEL Original File'd June 24, 1964F76./ //o 2 56.2 y 1o Lia "I 18/4 INVENTOR. 650266 5. Ail/M9 JTTOK/VE/J.

3,390,906 JOINT WITH INHERENTLY LIMITED TORQUE LEVEL George S. Wing,Torrance, Calif., assignor to-Hi- Shear Corporation, Torrance, Calil:'.,a corporation of California v Continuation of application Ser. No.377,761, June 24, 1964. This application July 31, 1967, Ser.'No. 660,161

. 6 Claims. (Cl. 287-18936) ABSTRACT OF THE DISCLOSURE 'A joint with aninherently limited torque level which comprises a workpiece having ahole 'theret'hrough, together with a fastener system which includes aheaded threaded pin and an inherently torque-limited nut. The pin fitsin the hole with the headbearing at one end, and with the nut-bearing atthe other. I a 1 The joint possesses inherent reliability and integritybecause a counterbore in the nut clears all incomplete threads on thepin adjacent to the workpiece, and makes a threaded engagement withthree fully-formed threads on the pin. v r

Optional features include the provision of a wrench engaging recess inthe threaded end ofthe pin which does not weaken the axial region wherethe three fully-formed threads are engaged, and a fiuid sealing jointwherein the pin makes an interference fit in the hole.

This invention relates to a joint with an inherently limited torquelevel which includes an inherently torque-limited fastener.

This application is a continuation of applicants copending applicationSer. No. 377,761, filed June 24, 1964, entitled Fastener Systems, nowabandoned, which in turn was a continuation-impart of applicants thencopending. application Ser. No. 129,699, filed Aug. 7, 1961,

entitled, Fasteners, now U.S. Patent No. 3,138,987,

issued June 30, 1964, whichin turn was a continuation-inpart'ofapplicants application Ser. No. 661,874, filed May 27, 1957, entitled,Fastener and Tool for Setting the Same, now abandoned, said Ser.No.'129,699 also being a continuation-in-part of applicants applicationSer; No.

793,861, filed Feb. 17, 1959, entitled, Fastener, now abandoned, whichis a division of applicants'application Ser. No. 643,183, filed Feb. 28,1957, how US. Patent No. 2,940,495, issued June 14, 1960, entitled, LockNut With Frangible Driving Portion. V V I Fastener systems wherein aportion of a pin or of a collar shears off at a predetermined torque areknown from the aforesaid patents, and from George S. Wing Patent No.3,041,902, issued July 3, 1962. They have in common the features of athreaded pin and a threaded collar, one of which has a portion ofreduced cross-sectional area which shears oif upon the application of atorque of given value. This shearing provides a visible indicationlthat'the fastener has been set to the given torque, and it alsoprevents additional torque from being applied to the system, because theportion which shears 3,390,906 Patented July 2, 1968 "ice of tightnesssuch as axial pre-load must have been achieved, and that this bepredictable from joint to joint. It is a fact that a given torque levelcan be reached without simultaneously attaining these other designlevels. For example, engagement by the nut with incomplete threads onthe pin can adversely affect the axial pre-load, making its levelunpredictable. As another example, penetration of wrench-engaging meansinto the pin to a region wherein a significant portion of the tensionload is taken up by the nut may seriously reduce the strength of thejoint. As another example, failure to provide three convolutions ofengaged fully-formed threads may seriously reduce the strength of thejoint. Thus even with a given torque level attained, other design levelswhich are just as necessary may be missed. However, the solecontrollable parameter is torque, so it is important to make the other'design levels predictable and attainable when the torque level isreached. It should be borne in mind that these joints are formed inproduction operations, and that there will therefore be a requirementfor'a substantial grip range, while still achieving these requirements.It is an object of this invention to meet these requirements.

' The joint produced is also adaptable to meet another productrequirement, namely for an inherently torquelimited joint which isfluid-tight.

It is an optional object of this invention to provide an inherentlytorque-limiting fastener system with inherent means for protectionagainst corrosion due to water, splash or submersion, thereby renderingthe system suitable for shipboard use even in submerged applications,and this without requiring any coatings or sealants. This also resultsin a joint with significantly higher allowable shear strength.

A joint according to this invention includes in combination a workpiecewith a hole therethrough in which there fits a pin which has alongitudinal axis and a circularly cylindrical shank having a firstdiameter. The pin also bears an exterior circumferential thread which isdisposed adjacent the first end of the pin and which has a seconddiameter, which is the largest diameter of the thread, this seconddiameter being no greater than the diameter of the hole. The .pin threadincludes at least one incomplete convolution and at least threefully-formed convolutions. In the assembled joint, all of the incompleteconvolutions stand outside the hole. Also, there is a wrench-engagingmeans which is formed at the first end of the pin.

The joint further includes an inherently torque-limited collar which isadapted to be threaded onto the pin, this collar comprising a body whichhas a, longitudinal axis and which has an axially-extendingpin-receiving hole therethrough. The collar includes a bearing sectionand a wrenchengaging section, there being a bearing surface on a firstend of the bearing section at a first end of the body, which bearingsurface lies substantially normal to the longitudinal axis.Wrench-engaging surfaces are provided on the wrench-engaging sectionadjacent to the second end of the collar. An internal thread is mediallydisposed in the wall of the pin-receiving hole and adapted to engage thethread on the pin. A counterbore bounds the hole contiguous to the firstend of the nut, this counterbore having a minimum aperture which isgreater than the second diameter.- The axial extent of the internalthread is such as to provide at least three complete convolutionsengageable by and engaged to three fully-formed threads of the pin overthe full grip range of the pin. The axial extent of the counterbore issuch that all of said incomplete threads are contained therein. Athreadclearance bore bounds the hole contiguous to thesecond end of thecollar and extends axially to the thread. It has a minimum aperturegreater than the said second diameter. A circumferential groove isformed in the body, the groove being disposed between the bearingandwrenchengaging sections. It provides a reduced cross-section that hasthe minimum torsional shear strength of the collar, whereby the maximumtorque which can be exerted between the sections is determined by thisgroove, the wrench-engaging section shearing off when torque of apredetermined value is exerted between the sections whereby the collaris shortened to the length of the bearing section in the assembledjoint.

According to a preferred but optional feature of the invention, thiscombination includes a workpiece having a hole therethrou gh with adiameter substantially such as to make an interference fit with thefirst diameter on the cylindrical shank of the pin. This interferencefit results in a fluid seal at the shank.

According to still another preferred but optional feature, thewrench-engaging means on the pin comprises a non-circular recessextending axially into the pin from the first end thereof, which recesspenetrates to a depth insutficient to reduce the cross-section of thepin within the three of said engaged fully-formed threads which areclosest to the incomplete threads.

The above and other features of this invention will be fully understoodfrom the following detailed description and the accompanying drawings,in which:

FIG. 1 is a side view partly in axial cutaway crosssection showing a pinaccording to the present invention;

FIG. 2 is a cross-section taken at line 22 of FIG, 1;

FIG. 3 is an axial cross-section of a collar according to the invention;

FIG. 4 is a right-hand end view of FIG. 3;

FIG. 5 is a cross-section taken at line 55 of FIG. 3;

FIG. 6 is a left-hand end vie-w of FIG. 3; and

FIG. 7 is an axial cross-section showing the fastener system accordingto the invention.

The drawings show the presently preferred embodiment of the invention.In FIG. 1, there is shown a pin 10 With a head 11, a shank 12 which iscircularly cylindrical, an external thread 13 and a first end 14. Theshank has a first diameter 15 and the thread has a second diameter 16,diameter 16 being the greatest diameter of any part of the thread, thesecond diameter being no greater and preferably somewhat less than thefirst diameter. The thread includes at least one imperfect convolution13a, as is common with rolled and machined heads. It also has at leastthree fully-formed convolutions 13b, as will be more fully disclosedhereinafter.

The pin has a longitudinal axis 17. At the first end there iswrench-engaging means which could be such as a stub which bears externalwrenching surfaces or the like, but which is preferably awrench-engaging recess 18. This recess extends axially into the pin fromthat end, This recess is preferably of the Allen-wrench type, iscentered on the axis and is symmetrical relative thereto. It extendsinto the pin to a depth to be discussed below. This recess isnon-circular in order that it may be engaged by a torque-type tool suchas an Allen head wrench (torque type tools hereinafter being genericallyreferred to as wrenches), so as to hold the pin against rotation whenthe fastener system is being set. The depth of the recess in the pin isindicated by dimension 19.

A collar is shown in FIG. 3. This collar includes a body 26 which has abearing section 27 and a Wrenchengaging section 28. These sections arespaced apart by a groove 29. The bearing section includes a bearingsurface 30 lying normal to axis 36 of the collar which is preferably,but need not be, a flat annulus, and is adapted to bear against aworkpiece 45 when the fastener is set. This bearing surface is disposedat the first end 31 of the collar. The external surface of the bearingsection is reduced by a taper 32 which serves to lighten the weight ofthe body. The bearing section includes a stiffening shoulder 33 which iscontiguous to groove 29, and terminates at the groove.

Wrench-engaging section 28 includes wrench-engaging means in the form ofsurfaces 35 which are preferably formed in a hexagonal array, althoughit will be understood that their basic characteristic is that of beingnoncircular so as to be engageable by a torque-applying wrench. Againthe term wrench is used generically for torque-applying tools.

Body 26 has a longitudinal axis 36 and also an axially extending hole37. This hole includes a counterbore 38 contiguous to the first end ofthe body. The depth of the counterbore will be discussed below.Contiguous to the counterbore there is an internal thread 39 adapted tomake a threaded fit with threads 13 of the pin. On the opposite side ofthread 39 from counterbore 38 and adjacent to second end 40 of thecollar there is a threadclearance hole 41.

counterbore 38 and thread-clearance hole 41 are most conveniently madecircular, although they could have other configurations if preferred.The minimum aperture of the counterbore is that of first diameter 15,and the minimum aperture of the thread-clearance hole is that of seconddiameter 16. The counterbore can thereby clear the projecting end ofshank 12 while receiving and containing the incomplete threads (see FIG.7). Threadclearance hole 41 clears the pin threads so as not to becomeengaged to them at all, thereby permitting the wrench-engaging sectionto fall free when the collar parts at groove 29.

Internal thread 39 extends axially through the stiffening shoulder 33and terminates short of the wrench-en gaging section. Groove 29 servesas the division point of the two sections, and is a convenient way offorming a region of minimum cross-sectional area axially even with thesaid groove.

Preferably, although not necessarily, the stiffening shoulder is pressedout-of-round after the nut is formed, so as to assume the oval shapebest shown in FIG. 5. When the collar is turned down onto the pin, theout-ofround structure is forced back into round by the pin, and thespringback force of the collar serves as a lock to hold the collar inits finally torqued condition on the pin.

FIG. 7 shows the presently preferred embodiment of the system includinga workpiece 45. As can be seen, the system has now been set and thewrench-engaging section has been torqued off by virtue of theapplication of sutficient torque to do the same. Suitable dimensions ininches for two fasteners of the type disclosed in the drawings areprovided in the following table:

The relationships inherent in the above dimensions and which provide theadvantages which are the objects of this invention are as follows.First, in a threaded engagement between a collar (nut) and a pin (bolt),of the load is taken up by the first three complete threads closest tothe workpiece. In this region, the pin should be solid across itscross-section. It therefore follows that where a wrench-engaging recessis used, it should not penetrate into the region of the three importantthreads for that specific installation. However, this is relativelydifficult to do, for the reason that as a matter of good industrialpractice, a pin should have a fairly broad grip range so that a minimumnumber of pin lengths need to be stocked.

In fields such as riveting, grip range is a relatively minor problem,because the plasticity of the fastener during setting accommodates to acertain degree. However, threaded fasteners are built to size and arenot plastically deformed at any time.

Furthermore, in threads on any pin, there will always be a thread or twoclosest to the shank which is or are incomplete (not fully formed),because of the way threads are made. It does no real good for the nut toengage these threads, because the results are unpredictable. A threadedfastener, to have grip range and the necessary strength, must thereforeprovide means for engaging three perfect threads of the collar and pinover a range as wide as from a nominally correct length. This is, inpart, accomplished by counterbore 28, which is of such length as toclear and receive without engagement the incomplete threads 47 of thepin, also clearing whatever portion of shank 12 projects beyond theworkpiece (as shown in FIG. 7) and by the threads in the collar being ofsuch length as to receive the three fully formed threads which it willengage when the fastener is fully set. The fasteners detailed aboveaccomplish this over a grip range of 1 The foregoing conditions existwith bearing surface 31 brought up against the workpiece, and with thejoint tightly set to its predetermined torque level. All incomplete pinthreads are disposed outside the workpiece; none is in the hole in theworkpiece. r

The cylindrical portion of the shank will preferabl make an interferencefit in the hole in the workpiece. An interference fit is properlyrepresented by a hole diametrically undersized by between 0.00l-0.0035"relative to the shank that is to be fitted into it. The pin may bepressed or pulled into the hole in accordance with known techniques.Were the threads on the pin to be pulled through in interference, theywould either be galled, or tend to ream out or scour the hole wall. Forthis reason, thread diameter 16 is made smaller than diameter 15 ofshank 12 by an amount at least sufficient to permit the thread to passfreely through hole 46. If fluid-sealing is not desired, and scouring orreaming is tolerable, then diameter 16 need only be no greater thandiameter 15. An interference fit is an example of a close tolerance fitwhich is customarily utilized in high-grade fasteners of the typeillustrated.

The depth of groove 29 is determined as a function of the torque desiredas a limiting value which in turn is a function of the strength of thematerials utilized and the cross-sectional area, and its placementrelative to the longitudinal axis.

This invention thereby contemplates a joint readily assembled withproduction techniques in which the imperfect threads common to threadedshanks have no effect on the reliability of the joint, in which threeengaged fully-formed convolutions provide reliable strength levels, andin which, when a wrench-engaging recess is used, the strength of thejoint is not adversely affected.

This invention is not to be limited by the embodiments shown in thedrawings and described in the description which are given by way ofillustration and not of limitation, but only in accordance with thescope of the appended claims.

What is claimed:

1. A joint assembled to a torque level that is inherently determined byone of its par-ts, comprising in combination: a workpiece having acircularly cylindrical hole therethrough with a diameter, a dimension oflength, and a bearing region surrounding one end of the hole; a pinhaving a longitudinal axis, a circularly cylindrical shank having afirst diameter disposed in the hole in the workpiece, and an exteriorcircumferential thread adjacent to a first end of the pin and having asecond diameter which is the largest diameter of the thread, the seconddiameter being no greater than the diameter of the hole in theworkpiece, the thread including at least one incomplete convolutionadjacent to the shank and at least three fully-formed convolutions, allof the incomplete convolutions standing outside the hole, andwrenchengaging means formed adjacent to the first end of the pin; and aninherently torque-limited collar threaded onto said pin, said collarcomprising a body having a longitudinal axis, and an axially-extendingpin-receiving hole therethrough, a bearing section forming part of thebody, a bearing surface on a first end of the bearing section at a firstend of the body which lies substantially normal to the longitudinal axisand bears against the said bearing region, an internal thread disposedin the wall of the pin-receiving hole within the bearing section, whichthread engages at least the said three fully-formed convolutions ofthreads on the pin, andv engaging the same, a counterbore bounding thehole contiguous to the first end of the collar, the counterbore having aminimum aperture greater than the said first diameter of the pin, and oflength receiving, and containing, all of said incomplete convolutions, awrench-engaging section forming part of the body and formed integrallywith said bearing section prior to formation of the joint,wrenchengaging surfaces on the wrench-engaging section adjacent to thesecond end of the collar, a circumferential groove formed in said bodyprior to formation of the joint, said grooves being disposed between thebearing and wrench-engaging sections and providing a reducedcrosssection with the minimum torsional shear strength of said collar,whereby the maximum torque which can be exerted between the sections isdetermined by the crosssection at this groove, the wrench-engagingsection having sheared off at the groove when torque of a predeterminedvalue was exerted between the sections thereby to leave the joint set toa torque level determined by said reduced cross-section, and the collarreduced in axial length to that of the bearing section.

2. A joint according to claim 1 in which the shank of the pin makes aclose tolerance fit in the hole in the workpiece.

3. A joint according to claim 1 in which the shank of the pin makes aninterference fit in the hole in the workpiece.

4. A joint according to claim 1 in which the wrenchengaging meanscomprises a non-circular recess extending axially into the pin from thefirst end thereof, which recess penetrates to a depth insufiicient toreduce the crosssection of the pin within the three of said engagedfullyformed thread convolutions closest to the incomplete threads.

5. A joint according to claim 4 in which the shank of the pin makes aclose tolerance fit in the hole in the workpiece.

6. A joint according to claim 4 in which the shank of the pin makes aninterference fit in the hole in the workpiece.

References Cited UNITED STATES PATENTS 1,971,200 8/1934 Proctor -12,430,613 11/1947 Hodge 85-45 2,895,367 7/1959 Nagy 85--l 2,940,495 6/1960 Wing 85-61 2,972,274 2/ 1961 La Bombard et al 851 3,041,912 6/1962Kreider et a1. 851 3,138,987 6/1964 Wing 85--61 CARL W. TOMLIN, PrimaryExaminer.

R. S. BRITTS, Assistant Examiner.

